(1) Field of the Invention
The present application relates to the field of rotorcraft, and it relates more specifically to a rotorcraft rotor comprising a hub made of composite material with the blades of the rotary wing of said rotor being mounted on the hub.
More specifically, the invention provides a rotorcraft rotor comprising a hub made of composite materials with the blades of the rotary wing of said rotor being mounted thereon.
(2) Description of Related Art
Rotorcraft are rotary wing aircraft having at least one rotor, including at least one main rotor of substantially vertical axis that provides the rotorcraft at least with lift.
In the specific context of a helicopter, said at least one main rotor provides the rotorcraft not only with lift, but also with propulsion in any direction of advance. The flight attitude of the helicopter may be modified by a pilot of the rotorcraft causing the pitch of the blades making up the rotary wing of the main rotor to vary cyclically and/or collectively.
Rotorcraft are also fitted with an anti-torque device providing with them with guidance in yaw, such as in particular at least one auxiliary rotor having a substantially horizontal axis. By way of example, such an auxiliary rotor may be a tail rotor or it may be formed by a propulsive propeller in the context of a helicopter having a high forward speed.
The blades of a rotorcraft rotor are conventionally mounted on a hub for driving them in rotation. The hub is mounted on a rotary shaft such as a mast for a main rotor, the shaft being driven in rotation by a power plant that supplies the mechanical power needed for operation of the rotorcraft.
Furthermore, the blades are individually mounted on the hub so as to be movable at least about a pitch variation axis, so as to enable a pilot of the rotorcraft to vary their pitch at least collectively, and possibly also cyclically, in order to modify the flight attitude of the rotorcraft.
With reference more particularly to a main rotor, the blades are mounted not only to be capable of pivoting about their pitch variation axis, but they are also commonly mounted to be movable with lead/lag motion and with flapping motion.
Under such circumstances, the blades are mounted more particularly so as to be movable relative to the hub not only about their pitch variation axes, but also about respective flapping axes oriented parallel to the general plane of a blade in question, i.e. mainly perpendicularly to the orientation of the plane of rotation of the hub, and about a respective lead/lag axis oriented perpendicularly to the general plane of the blade in question, i.e. mainly parallel to the orientation of the axis of rotation of the hub.
Concerning the orientations of the flapping axis and of the lead/lag axis, the main orientation specified relative to the orientation of the axis of rotation of the hub should typically be assessed relative to the ability of the blade to move in its own general plane about its various movement axes relative to the hub.
In this context, the mechanical strength of the hub of a rotorcraft rotor is naturally selected to withstand the forces that it needs to withstand in operation. Such forces are the result in particular not only of the centrifugal force generated by driving the rotor in rotation, but also of the freedom of the blades to move relative to the hub, at least about their pitch variation axes and possibly also, for a main rotor, about their flapping axes and about their lead/lag axes.
Traditionally, the hub of a rotorcraft rotor is made of a piece of metal.
Nevertheless, such a metal structure for the hub tends to give it considerable weight, and it is appropriate to reduce weight in the field of aviation. Furthermore, a metal hub needs to be machined in order to receive appropriately the various members providing the connection between the hub and the rotary shaft and also the movable connections between the blades and the hub.
It can thus be seen that although such a metal hub is satisfactory compared with respect to its function and in particular its strength, it nevertheless presents the drawbacks of being heavy for given volume, and expensive to obtain.
Such drawbacks can be limited in the specific context of a hub having flexible branches on which the blades are mounted so as to enable them to be movable relative to the hub. In this context, it is known to make a rotorcraft rotor hub out of composite materials that are made up of stacks of layers of mineral fiber fabric impregnated with a thermosetting resin.
Thus, the Document EP 2 234 880, equivalent to Document US 2011/116936, and Document U.S. Pat. No. 8,147,198, disclose a composite material yoke forming the hub of a rotorcraft rotor and its method of fabrication. Said yoke is made of layers of glass fiber fabric impregnated with a thermosetting resin and it includes respective blade-carrier branches.
The composite materials from which the yoke is made are used to advantage for providing the blades with freedom to move in flapping. More particularly, localized weakening of the branches of the yoke impart flexibility in flapping to the branches.
Proposals are also made in Document EP 0 221 678, equivalent to Document U.S. Pat. No. 4,714,409, to provide a rotorcraft rotor hub in which flexible branches for connecting the blades to the hub are made out of a resin having reinforcing plates embedded therein, which plates are made of composite materials. The composite materials used for forming flexible branches, or indeed the entire hub, are formed more specifically from layers of carbon fiber fabric impregnated with a thermosetting resin.
In the field of the invention, the following documents have also been consulted.
Document EP 0 120 803, equivalent to Document U.S. Pat. No. 4,568,244, describes a main rotor hub for a helicopter having a composite matrix of reinforcing fibers and epoxy. That hub made of composite material is provided with sockets, each having a radial thrust seat with a flat bearing surface for supporting a respective blade.
Document EP 0 340 095, equivalent to Document U.S. Pat. No. 4,915,585, describes a rotor hub made of composite material that is substantially cross-shaped, but without specifically describing the connection between its outer radial strength member and the margin of the hub.
Document FR 2 653 405, equivalent to Document U.S. Pat. No. 5,141,398, describes a lead/lag damper and resilient return device for rotorcraft rotor blades. Each blade of the rotor has a root whereby the blade is coupled to a hub that is itself driven in rotation about an axis of the rotor. A thin ring of viscoelastic material possesses internal and external walls that are bonded respectively to an internal strength member and to an external strength member. The external strength member presents a lever having hinged thereto via a ball joint a rigid rod that has a ball joint for constraining the rod to move with the root of the blade. The internal strength member is secured to a support that is driven by the hub.
Document GB 2 092 541, equivalent to Document U.S. Pat. No. 4,425,082, describes a composite hub for a helicopter rotor. That hub is made up of a plane plate having passing therethrough a series of openings that are uniformly distributed around its periphery. Each of the openings is suitable for receiving a flexible coupling providing a connection with an associated blade. The plate presents a laminated structure including a force distributor element in a middle layer. Radial leaf elements are arranged on either side of the distributor element and extend in planes perpendicular to the axis of rotation. Each annular leaf element is disposed on either side of the distributor element and comes into contact with the free ends of the associated series of radial elements.
Document U.S. Pat. No. 4,818,179 describes a hub for a helicopter rotor having a main retention plate. A peripheral ring of composite material presents reinforcing fibers that are oriented in the plane of the plate and perpendicularly to the retention plane. Other fiber reinforced rings are spaced apart perpendicularly to the central axis of the hub. Those rings form a flat tubular circumferential structure that surrounds the peripheral ring. An annular frame essentially comprises circumferential fibers that are interlinked with fibers that are inclined at a predetermined angle.
Document U.S. Pat. No. 5,478,204 describes a rotary wing aircraft rotor having a ring for supporting pitch bearings. That ring includes an upper cap and a lower cap. Various materials are proposed for the rotor, e.g. resin, epoxy, or reinforced thermoplastic resin.
Nevertheless, the conditions in which such composite materials are used for making the hub of a rotorcraft rotor having flexible branches that contribute to enabling the blades to move relative to the hub are limited in particular to rotorcraft of the light category. For rotorcraft of heavier categories in which the rotors support forces that are considered as being large, the blades are preferably mounted on a hub that is robust and rigid by means of a hinge system, such as a ball joint hinge providing the blades with freedom to move in multiple directions on the hub.
By way of example, such a ball joint hinge is provided by using a laminated elastomer body that incorporates metal laminations, commonly referred to as a “spherical thrust bearing”. Such an elastomer/metal laminated body is incorporated with an assembly strength member and is located between the hub and the blade root of a blade under consideration, being placed so as to bear against the hub in order to allow the blades freedom to move relative to the hub by virtue of the laminated body deforming.
It can be understood that in such a context of mounting blades on the hub by means of a hinge system, the material constituting the hub is conventionally selected to be a metal.
Using composite materials to form such a hub commonly involves the need to reinforce the hub against its own deformation under operating conditions.
Such reinforcement of the hub is commonly performed by placing a metal belt around the hub, which presents the main drawback of increasing the weight of the hub and of making it more complex to produce.
In addition, the volume of the hub needs to be increased in order to provide the looked-for robust mounting of blades with freedom to move relative to the hub by means of said hinge systems.
Such an increase in volume makes it necessary not only to extend the hub in thickness, where thickness is considered along the axially-extending direction of the hub, but also in diameter in order to impart robustness to the hub, enabling it to withstand the forces to which it is subjected in operation and relative to the ways in which it is connected firstly to the rotary shaft and secondly to the blades.